Therapeutic liver repopulation (TLR) is a process similar to bone marrow transplantation (BMT) resulting in replacement of >90% of hepatocytes in the recipient liver by transplanted donor cells. Although TLR to date has been achieved experimentally only in animal models, spontaneous liver repopulation is known to occur in humans. Therefore the procedure holds great promise for the treatment of human liver diseases, particularly hepatic enzyme deficiencies. Currently, the only cell source available for clinical liver cell therapy are hepatocytes isolated from the livers of beating-heart donors, the same source used to procure whole organs for orthotopic liver transplantation. However, liver stem cells may provide an alternative, more readily available source of hepatocytes. Our findings from the previous funding period make it unlikely that hematopoietic stem cells can be used for liver repopulation anytime soon. For this reason, we will refocus our efforts on the intrahepatic stem cell itself. Preliminary data indicate that hepatic progenitor cells are ischemia resistant and can survive for many hours after death. Cadaveric donors therefore may a useful source of liver repopulating cells. We have developed novel cell surface antibodies for the isolation of hepatocyte precursors from adult mice. The properties of these adult progenitors will be ascertained in vitro (gene expression profiles, clonogenicity, and expandability) and in vivo (repopulation ability) in our Fah knock-out liver repopulating assay. In Aim 1, we will study the oval cell response as a system to elucidate factors which control hepatic progenitor activation and then test these signals for their ability to expand progenitors from normal mouse liver in Aim 2. To translate insights gained by studying murine progenitor cells to clinical relevance, a robust system for xenogeneic repopulation of mouse liver with human hepatocytes is needed. In Aim 3, we will validate the Fah knockout model as a user-friendly system for this purpose. Relevance to public health: The studies proposed here will explore intrahepatic stem cells obtained from cadaveric donors as a potential cell source for liver cell therapy. These animal experiments will pave the way for the use of cadaveric human liver progenitors. In addition, the creation of a widely usable murine model for repopulation with human hepatocytes will facilitate translational studies in all areas of liver biology.